The present invention relates to the separation of finely divided powders into fractions with a specific size range and particularly to the separation of powders in the sub-micron size range.
Historically the separation by size of such very fine powders has been done using a sedimentation technique that relies on the differential rate of sedimentation of particles as predicted by Stokes' law. In practice the larger the particle, the faster it is deposited therefore if chemically identical powders with a range of sizes are dispersed in a dispersion medium in which the particles are not soluble, such as water, the heavier particles settle out first, followed by the next heavier and so on until the finest are deposited. The separation is usually carried out in a column and the sediment at the bottom of the column after all particles have settled out will have the finer particles collected in the top layers and the heavier particles in the bottom layers. This is sometimes accomplished in a cascade of separator columns each separating out a fraction of the particle sizes to minimize the waiting time. This can be quite considerable since sub-micron particles dispersed in water can take weeks to settle out. Working with a cascade of columns allows the larger sizes to be removed relatively quickly so as to leave the finer particle sizes behind.
In an attempt to speed up the process of settling out the finest particles, a settling agent of a lower density such as alcohol may be added to the dispersion medium and this is successful to some degree but the time taken to separate sub-micron particles is still very long. In addition the mixture then has to undergo dialysis to remove the settling agent before the powder is dried.
A further problem arises when drying the sub-micron particles. If the separation from the dispersion medium involves heating this causes the particles to agglomerate. There has been some success in drying the powders using a freeze drying technique. This reduces the agglomeration but does nothing to shorten the separation process.
Another separation technique involves elutriation in which a dispersion of the particles in, for example, water is cause to flow at a defined rate. This passes through a series of vessels of increasing diameter. The finer particles will travel further than the coarser such that a separation can take place. Once again however the problems of separation from the dispersion medium and drying discussed above are encountered.
A process for separating very fine particles has now been developed that can be completed in a fraction of the time taken using prior art techniques. Moreover the process allows rapid controlled drying without agglomeration. In this way the process of the invention represents an inexpensive, convenient and effective means for producing fine, and particularly sub-micron, powders from powder mixtures.